Muscles are a tractable system for studying cell determination and differentiation. Also, an understanding of muscle formation is important medically, since a significant portion of the U. S. population suffers from a muscle related disease or syndrome. My laboratory studies myogenesis in Drosophila embryos because it offers a powerful molecular and genetic system for identifying and characterizing the genes involved in muscle precursor commitment, differentiation, and fusion. The focus of our studies is the seven so-called neurogenic genes. These genes are thought to comprise a signal transduction pathway that allows initially equivalent cells to adopt different developmental fates. Best known for their role in the neuroectoderm, the neurogenic genes are also needed for muscle determination and differentiation. In neurogenic mutants, extra cells become muscle precursors and, later in development, defects in muscle organization and fusion are seen. These later defects could signify a second role for the neurogenic genes. Differences in both the early and late phenotypes of different neurogenic mutants suggest that some of the genes may act independently of the others, i.e., outside the common signal transduction pathway. To determine the functions of the neurogenic genes during muscle development, the muscle phenotypes of embryos that lack both maternal and zygotic neurogenic gene products will be analyzed using muscle-specific immunocytochemical markers. The phenotypes of different neurogenic mutants will be compared to determine whether the genes act in one or more than one pathway during muscle development. Second, the protein products of several neurogenic genes will be localized with respect to myogenesis. Last, mitotic clones will be made to determine which cells must express neurogenic genes for normal muscle formation. These experiments will clarify the roles of the neurogenic genes in muscle formation, and help identify the fate choices available to cells as they commit to a muscle lineage. As many neurogenic genes have been conserved through evolution, the results of these experiments should also help frame questions and experiments concerning the role of the neurogenic genes in vertebrate muscle formation.